Since magnesium is rich in resources among metals and has a weight less than that of aluminum and iron, attention is paid to the application thereof to light-weight parts. Although the actual strength of magnesium is inferior to that of aluminum and iron, it has high specific strength because it is light in weight, and thus it can be used in place of aluminum when the same strength is required. Accordingly, it is expected to apply magnesium to products such as a caring and welfare apparatus and the like the reduction of weight of which is required.
Further, magnesium has such excellent characteristics that it can prevent electromagnetic noise of electronic equipment because it is excellent in an electromagnet shielding property, has a superior capability for absorbing oscillation, and can reduce noise.
Moreover, magnesium has excellent resistivity against deformation and impact, whereas it can be machined easily. In addition to the above-mentioned, since magnesium has a low melting point and is excellent in a recycle property, it has superior characteristics in that it is a metal suitable for the environmental protection of the earth.
Incidentally, heretofore, magnesium metal products are manufactured by die casting and thixotropic molding (injection molding), and it is difficult to effectively manufacture the products by plastic working. Accordingly, magnesium cannot be molded without using a casting mold, and thus it is applied only to limited products. That is, at present, magnesium cannot establish a presence in the market in the application thereof to products for various uses made by plastic working because a technology for effectively manufacturing a magnesium metal thin plate has not been established.
Further, in the manufacture of the magnesium metal products described above, a lot of unnecessary products such as runner channels and the like are made in the process of molding magnesium in a metal mold, from which problems arise in that the yields of materials are deteriorated and gas pockets are formed in molded magnesium products by gas bubbles involved therein.
To cope with the above problems, Japanese Patent Application Laid-Open No. 2001-294966 discloses a method of manufacturing a magnesium metal thin plate that can be easily molded by manufacturing a magnesium metal sheet by a die cast method and then rolling the metal plate.
When the magnesium metal plate member is rolled as described above, voids, which were present inside of the plate member when it was cast, will be crushed and reduced in size or disappear. As a result, a magnesium metal thin plate without gas pockets therein can be obtained.
However, in the method disclosed in Japanese Patent Application Laid-Open No. 2001-294966, after the magnesium metal plate member is manufactured by the die cast method, it must be subjected to a trimming process once when it is rolled, and thus the plate member is rolled at room temperature.
That is, since the magnesium metal plate member is rolled at room temperature, it is pressed and deformed in a thickness direction by a rolling mill at such a draft as not to break the plate member. Since it means that the plate member cannot be deformed to a desired thickness that the draft is limited as described above, it cannot be said that the above method is excellent in productivity.
As a method of solving the above problem, it is also contemplated to hot-roll or to hot-extrude the plate member to deform it. However, an enormous amount of heat energy is consumed to execute the hot rolling or hot extrusion, from which a technical problem arises in that productivity is bad.
As means for solving the above problem, there is known a method of forming a continuous plate-shaped raw metal material through a continuous process by forming a metal slurry, which has a thixotropic property and contains a solid phase, by cooling magnesium molten metal and then further cooling and rolling the metal slurry (refer to Japanese Patent Application Laid-Open No. 2002-283007). In the method, however, since the magnesium molten metal is cooled to such a degree as to provide it with the thixotropic property and then supplied into a pair of rolls and cast thereby, the ratio of the solid phase in the molten metal supplied to the pair of casting rolls is high (20% or more, and a typical thixotropic property is ordinarily exhibited at about 50%). Since the magnesium molten metal, in which the solid phase and a liquid phase coexist as described above, has high viscosity, the molten metal is liable to be solidified in a nozzle for supplying it to the pair of casting rolls, and thus there is a large possibility that the nozzle is clogged by the molten metal. Accordingly, this method also has a technical problem in that yields in production are low and productivity is bad.